The invention relates to a polyacrylate which can be prepared by way of the conversion of nonpolar into polar functional groups by crosslinking by means of UV light and temperature, to a process for preparing this polyacrylate, and to the use of this polymer mixture as a pressure-sensitive adhesive composition.
In the field of pressure-sensitive adhesive (PSA) compositions, ongoing technological developments in the coating technique mean that there is a progressive need for new developments. Within the industry, hotmelt processes with solventless coating technology are of increasing importance in the preparation of PSA compositions, since the environmental regulations are becoming ever greater and the prices of solvents continue to rise. The consequence of this is that solvents are to be eliminated as far as possible from the manufacturing process for PSA tapes. The associated introduction of the hotmelt technology is imposing ever greater requirements on the adhesive compositions. Acrylic PSA compositions in particular are the subject of very intensive investigations aimed at improvements. For high-level industrial applications, polyacrylates are preferred on account of their transparency and weathering stability. In addition to these advantages, however, these acrylic PSA compositions must also meet stringent requirements in respect of shear strength and bond strength. This profile of requirements is met by polyacrylates of high molecular weight and high polarity with subsequent efficient crosslinking. These high shear strength, polar PSA compositions possess the disadvantage, however, that they are unsuited to the hotmelt extrusion process, because high application temperatures are necessary and because furthermore, the molecular weight of the polymer is reduced by shearing in the extruder. This damage significantly lowers the level of the adhesive properties. The bond strength and the tack are generally low, since owing to the polar fractions in the adhesive compositions the glass transition temperature is relatively high. The shear strengths of the hotmelt-coated acrylic PSA compositions, in particular, fall distinctly in comparison to the original, solvent-coated PSA composition. At the present time, therefore, different concepts aimed at reducing the flow viscosity and thereby facilitating extrusion coating of these PSA compositions are being investigated.
The industry is pursuing a variety of concepts for achieving this objective. One possibility is the highly efficient crosslinking of a low-viscosity, nonpolar acrylic adhesive composition only when it is actually on the backing. Acrylates containing electron-donating groups are copolymerized and, during crosslinking by UV or EBC (EBC: electron beam curing), they stabilize free radicals that are formed. Examples thereof are tertiary amine monomers [WO 96/35725], tertiary butylacrylamide monomer [U.S. Pat. No. 5,194,455] and the tetrahydrofurfuryl acrylates cited and used in EP 0 343 467 B1. A further efficient crosslinking concept is the copolymerization of UV photoinitiators into the polyacrylate chain. For example, benzoin acrylate has been used as a comonomer and the crosslinking has been conducted on the backing using UV light [DE 27 43 979 A1]. In U.S. Pat. No. 5,073,611, on the other hand, benzophenone and acetophenone were used as copolymerizable monomers.
An entirely new method has come from chip manufacture. By adding a photocationic initiator and carrying out irradiation with UV light, polymers containing ester functions are deprotected and free carboxylic acids or hydroxyl functions are generated. This method was described in U.S. Pat. No. 4,491,628. More recent developments have dealt with the change in the UV light wavelength to 193 nm [U.S. Pat. No. 5,910,392] and with the improvement in resolution [U.S. Pat. No. 5,861,231; U.S. Pat. No. 4,968,581; U.S. Pat. No. 4,883,740; U.S. Pat. No. 4,810,613; U.S. Pat. No. 4,491,628].
It is an object of the invention to provide a polyacrylate which can be prepared from a starting polymer of low processing viscosity and which is converted into a composition of high shear strength only after processing, e.g., in a hotmelt process. Through an appropriate choice of the reaction parameters during the conversion reaction, the intention is that it should be possible to adjust the physical properties of the polymer. The reaction causing the conversion should be able to be conducted when the polymer composition is already present in its ultimate form: for example, applied as an adhesive composition on a backing.
This object is achieved by means of a polyacrylate in accordance with the main claim. The subclaims relate to advantageous developments of the invention and to a process for preparing the polyacrylate, and also to its use.
The invention accordingly provides crosslinked polyacrylates prepared by crosslinking, induced by means of UV radiation, of a polymer mixture comprising the following components:
a) polyacrylate copolymers of the following monomers
a1) acrylic acid and/or acrylic esters of the following formula
CH2xe2x95x90CH(R1)(COOR2), 
where R1xe2x95x90H or CH3 and R2 is an alkyl chain having 1-20 carbon atoms, at 70-99% by weight, based on component (a),
a2) olefinically unsaturated monomers containing functional groups, at 0-15% by weight, based on component (a),
a3) tert-butyl acrylate,
at 1-15% by weight, based on component (a), and
b) a photocationic initiator
at 0.01-25% by weight, based on the overall polymer mixture.
In a particularly advantageous embodiment, component (a1) is present at 85-89% by weight, based on component (a), and/or component (a2) is present at 4-6% by weight, based on component (a), and/or component (a3) is present at 7-9% by weight, based on component (a).
It is very advantageous to use a polymer mixture to which up to 3% by weight, based on the overall polymer mixture, of
c) a difunctional or polyfunctional crosslinker is added.
Crosslinkers which can be used here are all difunctional or polyfunctional compounds whose functional groups are able to enter into a linking reaction with the polyacrylates, especially addition polymerization, polycondensation or polyaddition reactions. Preferably, these reactions will take place on a carboxyl group. Particularly suitable crosslinkers are epoxides or isocyanates containing at least two functional groups, although all other compounds which react with carboxyl groups are also suitable. It is also possible to use metal chelate compounds for this purpose.
The invention further provides a process for preparing a polyacrylate, which comprises applying the polymer mixture to a backing material and crosslinking it on said backing material by irradiation with UV light.
The invention further provides for the use of the polyacrylate as claimed in any of the above claims as a pressure-sensitive adhesive composition, especially its use as a pressure-sensitive adhesive composition for an adhesive tape, where the acrylic pressure-sensitive adhesive composition is present as a single- or double-sided film on a backing sheet.
As backing materials, for adhesive tapes, for example, it is possible in this context to use the materials customary and familiar to the skilled worker, such as sheets (polyester, PET, PE, PP, BOPP, PVC), nonwovens, wovens and woven sheets, and also release paper if desired. This list is not intended to be conclusive.
The invention set out above makes it possible to achieve the stated objects. A polymer mixture is presented which is present in low-viscosity form during processing and in which the shear strength can be increased after it has been applied, for example, to a backing, this increase in shear strength being achieved by converting nonpolar into polar groups. The flow viscosity of the starting polyacrylates is very low and is therefore highly suitable for the hotmelt process, for example.
The principles of the invention are set out below. The starting polymers may be prepared by a free radical polymerization, and the molecular weight is of the order of 1,000,000. For the starting polymer mixture it is possible to use any polymer which has adhesive properties in accordance with the Handbook of Pressure-sensitive Adhesives, p. 172, xc2xa71, 1989. As an essential component for the deprotection reaction which is to take place subsequently, polymers prepared using tert-butyl acrylate as comonomer must be present, since the conversion of the nonpolar into polar groups is based on the replacement of the tert-butyl group by protons.
The chemical reaction whose consequence is to convert nonpolar into polar PSA compositions on the backing is a deprotection of the tert-butyl acrylate to give the free carboxylic acid, in the presence of protons, and has already been known for a long time [Loev, Chem. Ind. (London) 193 (1964)]. 
This reaction is used in the photoresist technology (photoresists: light-sensitive, film-forming materials whose solubility behavior alters by exposure to light or other irradiation; in the case of negative working photoresists, this takes place by crosslinking or photopolymerization) for the deprotection of tert-butyl groups. In that technology, the acid is produced by the UV irradiation of an admixed photocationic initiator.
In the context of this invention, this technology has been transferred to PSA compositions.
Cationic photoinitiators are typically aryidiazonium salts (xe2x80x9conium saltsxe2x80x9d) which may be represented in general by the formula Arxe2x80x94Nxe2x95x90N+ LXxe2x88x92, where LXxe2x88x92 is an adduct of a Lewis acid L and a Lewis base Xxe2x88x92. Particularly advantageous for LXxe2x88x92 are BF4xe2x88x92, SbF5xe2x88x92, AsF5xe2x88x92, PF5xe2x88x92, SO3CF2xe2x88x92. Under the influence of UV radiation, there is rapid cleavage of the molecule into the aryl halide (ArX), nitrogen, and the corresponding Lewis acid. Diaryliodonium salts (C6H5)2I30 LXxe2x88x92 and triarylsulfonium salts (C6H5)3S+ LXxe2x88x92 are also known for use as cationic photoinitiators; in the presence of proton donors, they form strong (Brxc3x6nstedt) acids which are likewise highly suitable for the initiation of cationic polymerizations.
Sulfonium salts as cationic photoinitiators are also present, for example, in the form of the compounds H5C6xe2x80x94COxe2x80x94CH2xe2x80x94S+ LXxe2x88x92 or H5C6xe2x80x94COxe2x80x94CH2xe2x80x94Pyr+ LXxe2x88x92, in which Pyr is a nitrogen-containing heteroaromatic system (e.g. pyridine, pyrimidine).
For the crosslinking reaction depicted, the photocationic initiators familiar to the skilled worker are used, preferably one of the initiators from the group set out in the above paragraph.